EXTRUSION PROCESSING OF GRANULAR STARCH-G-POLY(METHYL ACRYLATE) - EFFECT OF EXTRUSION CONDITIONS ON MORPHOLOGY AND PROPERTIES

Starch-g-poly(methyl acrylate) copolymers (S-g-PMA) containing 10, 30, 46, and 58% PMA, by weight, were prepared by ceric ammonium nitrate-i nitiated polymerization of methyl acrylate onto granular cornstarch. G raft copolymers were extrusion-processed through a strand die with wat er contents of 10 and 30% (based on starch) and at temperatures of 140 and 180-degrees-C, and the resulting strands were then pelletized. Pr operties of ribbons prepared from extrusion-processed S-g-PMA depended on the combined effects of processing temperature, PMA level in the g raft copolymer, and water content during extrusion. Ribbon formation w as poor at the lowest PMA level of 10%, and extruded ribbons did not h ave sufficient continuity for tensile testing. Ribbons with smooth sur faces were obtained with 10% water in the polymer, whereas a water con tent of 30% produced extrudates that were rough and pebbly. When proce ssed with 10% water, graft copolymers containing 46 and 58% PMA exhibi ted the highest values for both ultimate tensile strength (UTS) and % elongation at break (% E) when the extrusion was performed at 180-degr ees-C. UTS was reduced and % E was increased when the water content du ring processing was increased to 30%. At the highest grafting level (5 8%), discrete granules of S-g-PMA were still apparent in scanning elec tron micrographs of extruded ribbons. More granule disruption occurred when the PMA level was reduced to 46%, and virtually none of the orig inal granule structure remained after extrusion with 30% water. The ef fect of water content during processing was especially apparent at the grafting level of 30%, and extrudate properties were consistent with the formation of starch as the continuous phase under high moisture co nditions. The behavior of S-g-PMA granules during extrusion can be exp lained by cross-linking within the starch matrix during the graft poly merization reaction. DSC data were consistent with this theory.